Geodiversity of Applecross: Statement of Significance and Identification of Opportunities Geology and Landscape (Northern Britain) Programme Report OR/07/020 BRITISH GEOLOGICAL SURVEY GEOLOGY AND LANDSCAPE (NORTHERN BRITAIN) PROGRAMME REPORT OR/07/020 Geodiversity of Applecross: Statement of Significance and Identification of Opportunities K M Goodenough and A Finlayson The National Grid and other Ordnance Survey data are used with the permission of the Controller of Her Majesty’s Stationery Office. Licence No: 100017897/2005. Keywords Applecross; geodiversity. Front cover View north-east along the shoreline of Applecross village. Jurassic limestone in the foreground; Applecross Formation sandstones form the glaciated hill of Beinn a’Chlachain in the distance. Bibliographical reference GOODENOUGH, KM AND FINLAYSON, A. 2007. Geodiversity of Applecross: Statement of Significance and Identification of Opportunities. British Geological Survey Open Report, OR/07/020. 18pp. Copyright in materials derived from the British Geological Survey’s work is owned by the Natural Environment Research Council (NERC) and the authority that commissioned the work. Maps and diagrams in this book use topography based on Ordnance Survey mapping. © NERC 2007. All rights reserved Keyworth, Nottingham British Geological Survey 2007 BRITISH GEOLOGICAL SURVEY The full range of Survey publications is available from the BGS British Geological Survey offices Sales Desks at Nottingham, Edinburgh and London; see contact details below or shop online at www.geologyshop.com Keyworth, Nottingham NG12 5GG The London Information Office also maintains a reference 0115-936 3241 Fax 0115-936 3488 collection of BGS publications including maps for consultation. e-mail: [email protected] The Survey publishes an annual catalogue of its maps and other www.bgs.ac.uk publications; this catalogue is available from any of the BGS Sales Shop online at: www.geologyshop.com Desks. The British Geological Survey carries out the geological survey of Murchison House, West Mains Road, Edinburgh EH9 3LA Great Britain and Northern Ireland (the latter as an agency 0131-667 1000 Fax 0131-668 2683 service for the government of Northern Ireland), and of the e-mail: [email protected] surrounding continental shelf, as well as its basic research projects. It also undertakes programmes of British technical aid in geology in developing countries as arranged by the Department London Information Office at the Natural History Museum for International Development and other agencies. (Earth Galleries), Exhibition Road, South Kensington, London SW7 2DE The British Geological Survey is a component body of the Natural Environment Research Council. 020-7589 4090 Fax 020-7584 8270 020-7942 5344/45 email: [email protected] Forde House, Park Five Business Centre, Harrier Way, Sowton, Exeter, Devon EX2 7HU 01392-445271 Fax 01392-445371 Geological Survey of Northern Ireland, Colby House, Stranmillis Court, Belfast BT9 5BF 028-9038 8462 Fax 028-9038 8461 Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB 01491-838800 Fax 01491-692345 Columbus House, Greenmeadow Springs, Tongwynlais, Cardiff, CF15 7NE 029–2052 1962 Fax 029–2052 1963 Parent Body Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, Wiltshire SN2 1EU 01793-411500 Fax 01793-411501 www.nerc.ac.uk Contents Contents...........................................................................................................................................i 1 Introduction............................................................................................................................ 1 2 The Geological History of Applecross.................................................................................. 1 3 Geology and Landscape of Applecross................................................................................. 7 3.1 Ardheslaig to Kenmore................................................................................................... 7 3.2 North-western Applecross.............................................................................................. 8 3.3 The Applecross Hills ...................................................................................................... 9 3.4 Applecross Village........................................................................................................ 10 4 Opportunities for geological interpretation around Applecross ..................................... 11 4.1 Interpretive opportunities for Applecross village......................................................... 11 4.2 Interpretive opportunities for Walkers, Cyclists and Drivers....................................... 11 5 Further Reading................................................................................................................... 12 i 1 Introduction This report has been prepared for the Applecross Estate Trust, in order to provide a summary of the geological history of Applecross, to identify the key geodiversity features, and to suggest opportunities for geological interpretation. The geology of Applecross can be divided into four main elements. These are: the Lewisian gneisses, the oldest rocks in Britain, which are exposed on the peninsula’s north coast; the Torridon Group sandstones, which make up the hills that form the main part of the Applecross peninsula; the younger sedimentary rocks around Applecross village itself; and the glacial features formed during the last Ice Age. Each of these four elements represents a different part of the Earth’s history, and they provide a fascinating record of the changing environment and land surface of western Scotland. The first section of this report describes the geological history of Applecross in more detail, and emphasises some of the important events. The second section discusses the relationship between geology and the landscape of Applecross, and focuses on some areas and localities that are of particular importance in terms of their geological interest. Possible interpretation opportunities and suggestions for geotourism facilities are given in the third section. 2 The Geological History of Applecross The oldest rocks on the Applecross Peninsula are the Lewisian gneisses, which are exposed along the peninsula’s north coast, around Kenmore and Ardheslaig. Recent dating work, involving separation of particular minerals from within the gneisses and very careful measurement of radioactive elements with known decay rates in those minerals, has dated gneisses from the Loch Torridon area at more than 3,100 million years old – making them the oldest known rocks in Britain, and among the oldest rocks in Europe. Structures within these rocks record a long and complex history, which is extremely difficult for geologists to unravel. The early part of the Earth’s history – the Precambrian Era – has always proved difficult for geologists to understand in detail. By 3,100 million years ago, it is thought that the processes of plate tectonics had begun to operate, and so continents moved slowly across the surface of the globe, colliding with each other to throw up mountain ranges (as with the present-day Himalaya) or splitting apart to produce new oceans (as in the present-day Red Sea). The surface of the Earth would have looked very different to today; the only life-forms were simple clusters of cells floating in the oceans, and there was little or no oxygen in the atmosphere. The oldest parts of the Applecross Lewisian gneisses were formed at depths of several kilometres in the roots of an ancient continent, where magma (molten rock) slowly cooled, crystallised and solidified to form igneous rocks similar to granites. These rocks remained within that ancient continent for hundreds of millions of years, until eventually two continents collided, burying the igneous rocks at depth beneath a mountain range. The rocks were buried so deeply that they were heated up significantly, and these higher temperatures led to the formation of new minerals in the rocks through the process of metamorphism. Growth of new minerals under pressure meant that the rocks developed a banded appearance – the characteristic feature of the metamorphic rocks known as gneisses. At a later stage, further compression of these rocks at great depths meant that the banding became crumpled and folded. 1 Around 2400 million years ago, a change in the forces acting on this piece of continental crust meant that it began to stretch and rift apart, and magma from deep within the Earth was forced up cracks in the crust. This magma eventually cooled in those cracks, forming dykes – narrow, near-vertical sheets of dark-coloured rock. Similar dykes occur in the Lewisian gneisses throughout the North-west Highlands, and are known as the Scourie Dykes. The dykes would once have cut straight across the banding in the gneisses, but that relationship has been destroyed by another episode of continental collision that occurred around 1800 million years ago, once again burying the gneisses and creating new structures within the rocks. It was at this time that the gneisses of Applecross became joined to other gneisses in the Gairloch area. Even although the rocks of the two areas had substantially different histories up until 1800 million years ago, those histories are not easy to unravel, and so geologists have grouped all the Precambrian gneisses of Scotland’s north-west coast, and of the Outer Hebrides, together under the name of the Lewisian Gneiss Complex.